Some basic science here.....
So Model A brakes properly rebuilt do a wonderful job stopping it from 65-70 MPH. The limiting factor is the tire contact patch on the pavement.
The best you can do force wise is keep the braking force just below where the wheel locks up. Once it locks up the braking force is greatly reduced. Factory brakes have no problem locking the wheels when they are rebuilt properly.
In all the posts I've seen about mechanical vs hydraulic (or in this case disk) brakes, IMO, this is the crux of the matter.
As has been previously stated, for normal daily driving, properly maintained Model A brakes are fine. Occasionally, however, our brakes are called upon to stop a vehicle as quickly or in as short a distance as possible to prevent rear-ending another vehicle or running into someone or something. In these situations, modulation is key. Modulation is being able to feel the braking effort and modulate the brakes to the threshold of skidding, without skidding.
For some this would make no difference, being in a panic situation would just mash the brakes, hold on, and hope for the best. For those who know how to feel and modulate the brakes the difference between hydraulic and mechanical brakes makes a very large difference.
I’m an avid cyclist and have ridden many 10’s of thousands of miles, both mountain/trail, and road. Mountain bikes have used mechanical or hydraulic disk brakes for years but the road bike industry has only recently started using mechanical or hydraulic disk brakes, relying on the age-old rim brakes. When I bought a new road bike with disk brakes I noticed the feedback and modulating abilities on the first ride. The ability to modulate the brakes has on more than one occasion saved me, and with hydraulic disks, it’s much more positive and controllable than rim brakes.
I would be very interested in seeing two similar Model A’s (same body style tires, weight, and preferably driver) one with properly maintained/adjusted stock brakes and the other with Hydraulics, tested for stopping distance at various speeds.
This may be something I’ll try in the next couple of years as the PU I’m currently putting together has relatively new, stock brakes, but I have a Clings hydraulic conversion I planned on putting on it. We’ll see, no promises.
Tom